The present invention relates to a method and apparatus for use in a protective relay system including a digital computer for protecting an electric power system.
In a protective relay system including a digital computer, AC electrical quantities such as voltage and current of the electric power system are sampled and processed. It is sometimes required to obtain data sampled at different phase angles. For instance, squares of two data sampled at phase angles 90.degree. apart from each other are added to determine the square of the amplitude of a current. Where the sampling interval is 30.degree., the data i.sub.m obtained at a sampling time point and the data i.sub.m-3 obtained at a sampling time point preceding the first mentioned sampling time point by three sampling intervals. These data are used to calculate the square of the amplitude I in accordance with the following equation: EQU I.sup.2 =i.sub.m.sup.2 +i.sub.m-3.sup.2 ( 1)
It should however be noted here that equality of the difference between the sampling time points of the two data i.sub.m and i.sub.m-3 with 90.degree. is maintained on condition that the frequency of the detected current and the frequency of an oscillator used to determine the sampling interval have a constant relationship between them. Calculation in accordance with the equation (1) is affected, for example, by variation of the frequency of the detected current. If the variation of the angular frequency is .DELTA..omega., then ##EQU1## where I.sub.o represents a true value of the amplitude of the current. From the equations (1) and (2), we obtain: ##EQU2## It will be understood that I signifies the calculated value of the amplitude of the current.
The third and fourth terms cos.sup.2 .DELTA..omega./.omega..multidot..pi./2 in the second term of the equation (3) are error factors. If, for example, .DELTA..omega./.omega.=0.1, then: EQU cos.sup.2 .DELTA..omega./.omega..multidot..pi.2=0.976 EQU sin .DELTA..omega./.omega..multidot..pi.=0.309 EQU sin.sup.2 .DELTA..omega./.omega..multidot..pi.2=0.0245
It will be seen that the third term predominates among the factors of the error. With the variation of the frequency of 10%, the error in I.sup.2 as calculated in accordance with the equation (1) is, at the most, in the order of 16%. The error in I is, at the most, about 7%.